When an external dpc becomes an internal wall, and dpc with different ground levels?

[surrey_100e]

Hi, I’m just about to start a small lean too side extension and have a couple of questions about dpc levels if anyone can please advise, before I consult the bco, I want to half a bit of an idea what I’m talking about.

The house was built in the 1960s into a hillside on chalk and the back of the house and garden are higher than the front by just over a meter. It has cavity walls, back of the house that’s below ground levels appears to have render slate render and slate to stop the damp and that appears to have worked.

Questions
-Where the existing side wall will become an internal wall, does the new dpm need to lap up past the original dpc then get battened, insulated and plasterboarded? You can see the upper original dpc is stepped all the way down to the correct level, there is a lower dpc but that runs back up to the upper after a short distance.

-I’m building a retaining wall at the back similar detail to the below drawing, would I need a vertical dpc between the original wall and new wall below the ground level?

- On the opposite side where the new wall will be there is a sloped footpath that goes to the upper ground level. This will require a stepped external dpc 150mm above the footpath, the internal dpc will be at the required finished floor level. Would I need a cavity tray as only seem to see this detail where the external ground level is lower than the internal floor not higher?

Thanks
Martin

 

[^woody^]

Best put the vDPM up the outer side of the internal leaf, or the inner side of the outer leaf.

 

[surrey_100e]

Thank you.

Would you suggest using a liquid type dpm, otherwise I’m thinking how would I use wall ties without puncturing any type of membrane?
Or do something like described here https://www.diynot.com/diy/threads/internal-floor-level-below-ground-level.357092/page-2

Would anything be required between the existing wall and the new wall or just build up to it?

I found the below for possible wall detail where existing dpc is. Thoughts?

Thanks again